In electric motors, torque is approximately proportionate to the product of current and magnetic flux density. In turn, two primary loss components exist which are related to these two quantities. The current-related loss component is due to current flow through conductors (e.g., losses within windings and rotor bars); this loss component is proportionate to the square of the rms current. The second loss component physically takes place in magnetic core elements such as the laminations and is approximately proportionate to the square of the product of magnetic flux density and electrical frequency. Two key consequences of these relations are first that energy efficiency is optimized at points of operation where the conductor and magnetic losses are approximately equal; and second that through-power can be increased without loss of efficiency provided speed (electrical frequency) is maintained proportionate to torque.
As speed and torque are increased, heat dissipation increases. Therefore improved cooling methods are required to limit temperatures to required values. In the case of induction motors this is a particular challenge as a significant fraction of the total heat dissipation physically occurs within the rotor due to the I2R losses associated with the rotor bars and end rings. Air cooling generally becomes insufficient when heat flux values exceed associated thresholds. Unfortunately, liquid cooling techniques for such rotors have proved cumbersome in the past due to problems associated with transferring fluid flow between rotating and non-rotating members. Additional problems exist, such as preventing the radial air gap between the rotor and stator from flooding with coolant as this greatly adds to drag loss at high speeds. Other challenges with liquid cooling include ease of mechanical assembly, uniformity of cooling, prevention of air entrapment in the coolant, and in some cases, the need for insulating the rotor and stator from the housing.